1. Field of the Invention
This invention relates to new and useful high strength fibers of arylene oxadiazole/arylene N-alkylhydrazide copolymers. The fibers are particularly useful in reinforcing applications such as in tire cord.
2. Description of the Prior Art
Imai, Journal of Applied Polymer Science, Vol. 14, pages 225-239 (January, 1970), describes the preparation of fibers from sulfuric acid solutions of p-phenylene oxadiazole/m-phenylene oxadiazole copolymers. Imai also describes unsuccessful attempts to prepare useful fibers from sulfuric acid solutions of poly-p-phenylene-1,3,4-oxadiazole.
U.S. Pat. No. 3,536,651 teaches the preparation and conversion of poly(terephthalic hydrazide) fiber to poly-p-phenylene-1,3,4-oxadiazole fiber by using the process described in U.S. Pat. No. 3,238,183, that is, by heating the polyhydrazide fiber to effect intra-linear dehydration-cyclization of the hydrazide moieties to oxadiazole moieties. According to the patent disclosures poly(terephthalic hydrazide) polymer is prepared in an N-substituted amide solvent, such as hexamethylphosphoramide, and is solution spun into fiber using dimethyl sulfoxide as the spinning solvent.
U.S. Pat. No. 3,642,708 teaches a process for the fabrication of shaped articles of poly-p-phenylene-1,3,4-oxadiazole by using an oxadiazole-N-methylhydrazide copolymeric resin as a precursor. The resin is prepared in fuming sulfuric acid and is stated to be significantly more soluble in common N-substituted amide solvents than is poly(terephthalic hydrazide). The process involves shaping an article of the copolymeric resin using a solution of the resin in an organic polar solvent, then removing the solvent by drying and finally heating the shaped article to between 220.degree. C. and 350.degree. C. to convert the N-methylhydrazide units to oxadiazole units: ##STR1## While the copolymeric resins illustrated in U.S. Pat. No. 3,642,708 may be useful for preparing films and coatings, the resins are not of sufficient molecular weight for preparing high strength fibers.